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Adorable story alert!
So I was hanging out with my two year old second cousin
in the ocean in Florida.
And we're jumping up and down with the waves and it's fun,
but then the water got all calm and I said, "Oh, no more waves!"
Because, like, that's the top tier of communication I'm going for
with a 2 year old, but then he said, "Don't worry, there will be more."
And I was like "How do you know." And he was like "It's a Cycle."
Yes! It is a cycle! The earth is filled with cycles...
and if my two year old second cousin knows it, you should know it too!
The universe is the great recycler.
All the stuff that we've got on Earth, every last particle
of matter or calorie of energy, has been around since the Big Bang.
It just keeps getting repurposed over and over again.
And when it comes to matter at least,
Earth is essentially a closed system.
All matter gets passed around in continuous biogeochemical cycles,
which are pathways for molecules like water or elements like carbon,
nitrogen and phosphorous to move through all of the earth's
various ecological and geological compartments.
Now, of course we couldn't possibly talk about how all matter gets
cycled around Earth in one video, because the earth is pretty big.
But consider this an introduction to biogeochemical cycles,
starring my two personal favorites...carbon and water.
Now, I'm sure you already know about at least one kind of
planet-wide recycling, because it's the most obvious to us:
the hydrologic cycle, which describes how water moves on, above,
and below the surface of the Earth,
driven by energy supplied by the sun and the wind.
In talking about the hydrologic cycle, it's most useful
to think about all the water on Earth being held
in a series of reservoirs: the ocean, for instance,
or the atmosphere in the form of clouds, or in polar icecaps.
So not only does water cycle through different places,
it also takes different forms at different places in the cycle:
liquid, solid or gas.
And since it's a cycle, there is no beginning and there is no end,
so where we start our discussion is arbitrary...
but we're going to start it off with precipitation:
rain, hail, snow, sleet, graupel... all that stuff is precipitation.
It happens when water that's being held in the atmosphere
condenses, or turns from gas into a liquid,
and then occasionally freezes into a solid, right up in the air.
The opposite of condensation, of course, is evaporation,
the conversion of a liquid into gas.
And when a substance converts straight from
a solid to a gas, that's sublimation.
And when it's from a gas to a solid...that's deposition.
And now you know! But back to condensation.
It's responsible for the formation of clouds,
which happens when air containing water vapor rises and cools,
or is compressed to the point that it can no longer be a gas.
At this point, the vapor forms droplets.
This is the same thing you see happening on your glass
of iced tea on a humid day: the water in the air around
the glass gets cold and turns from gas into liquid.
So, a cloud is just a big pile of condensed water droplets.
In a sense, it's a gigantic, floating reservoir.
Clouds are a handy feature of the hydrologic cycle because
as they drift over the landscape, they move water around
the globe, so water that evaporates over the ocean
can be deposited somewhere else.
Otherwise, if water always got deposited right where it
evaporated, the precipitation would be almost all right
over the ocean, because that's where most of the evaporation
on Earth takes place.
So, wind moves clouds, and as water keeps condensing,
clouds get heavier and heavier until our old friend
gravity takes over and pulls the condensed droplets to the
ground in the form of rain.
Or in the form of snow or hail or sleet or graupel.
So now the water's on the ground, but gravity continues to work on it,
pulling it toward it's resting place, whatever that might be.
It either pulls the water across the surface
of the land toward the lowest point, in a process
called runoff, or it pulls it underground.
Water can be trapped or stored for a little while in places
like lakes, ponds, and wetlands, but most of the water
that falls as precipitation gets pulled lower and lower and lower
as runoff through the creeks,
streams and rivers until it reaches the oceans.
Now, in really cold places, water of course freezes,
and hangs around as ice in certain places for thousands of years
at a time, like at the poles, in glaciers, and on mountaintops.
But when it melts, most of it, too, runs off into the oceans.
So, you see where this is going: Oceans are a big deal.
They're pretty much the biggest deal.
They're the reason that we have the hydrologic cycle
in the first place.
They're also the reason we have awesome stuff like weather
and life on Earth.
The weird thing about oceans though is that they're salty.
And there's a reason for this!
As water runs to the ocean, it erodes minerals like salt
from soil and carries it to the ocean.
Now water heading to the ocean might not taste salty,
but the salt's in there.
But here's the thing, when the water evaporates again,
the salt doesn't evaporate with it. It get's left behind.
You keep this up for a few billion years,
with pure water evaporating from the ocean and then returning
with tiny amounts of salt, and that's your recipe for
a billion cubic kilometers of brine.
And all of this shows that the world's oceans are literally
the last stop for all the liquid water on Earth.
The only way to get out of there is through evaporation,
and that leaves all your minerals behind.
Now, living things also have their role to play
in the hydrologic cycle.
In both plants and animals, the breakdown of carbohydrates
to produce energy produces water as a waste product.
So we lose water through evaporation from our skin,
we also exhale water vapor, and of course we pee it out.
Indeed, most organisms on earth are made mostly of water,
although that water cycles in and out of us pretty quickly.
In plants, water is sucked up through the roots and moves up to the
the leaves, the gas exchange organs, where it evaporates quickly.
This process is called evapotranspiration, and since there
are so many plants here on Earth, it's responsible for a
good amount of the water that enters the atmosphere.
This process is essentially the opposite of condensation,
in that it turns liquid water into gas.
The energy of the sun drives evaporation, whether it's
from the surface of the ocean or from treetops and leaves.
And then once all that water evaporates into the atmosphere,
we're right back where we started!
It's a cycle!
So, now that you know a little bit about the hydrologic cycle,
it's a little easier to understand how the carbon cycle works.
Carbon is one of the most abundant elements in the universe,
and here on Earth, it's always on the move, just like water,
jumping from one reservoir to the next.
And that's a good thing, because
A) all living things require carbon for their structure
and to fuel their bodies,
and B) it's a big component in a bunch of nonliving things
as well: it's in rocks, in the ocean, trapped in ice,
plus it's in the atmosphere, where it helps regulate the temperature.
Without carbon dioxide, Earth would basically be a frozen wasteland.
So lucky for us, there's a whole pantsload
of carbon out there, because we need it.
Let's start out with the carbon in living things.
If you were to take all the water out of your body,
carbon would constitute about half of what remained
in the little pile of dust that used to be you.
And the first biological carbon reservoir is plants.
They absorb a bunch of carbon dioxide out of the atmosphere
because they need it to photosynthesize.
But CO2 is also one of the byproducts of respiration,
a process by which they use that energy.
So, plants take in carbon dioxide from the atmosphere
during photosynthesis, and then release CO2 back out
into the atmosphere during their respiration process
to make ATP for all their cellular functions.
And right now you're like,
"waywaywaywait... isn't the deal that plants get to take in
the carbon dioxide and animals get to breathe it out?"
Well, yes and no.
It's just that plants take in more CO2 from the atmosphere
than they give off through respiration.
The rest is, like, their profit.
It's what becomes the body of the plant.
That's right. That big old massive tree.
All of that mass came from gas. Pretty cool.
So, carbon absorbed by plants has three possible fates:
it can be respired back into the atmosphere,
it can be eaten by an animal,
or it can be present when the plant dies.
And if a tree falls in the right kind of forest
and it's not allowed to decompose normally because a bunch
of other plants all fell right on top of it, and they die,
and get buried, and squish together and form rocks like coal.
We call these carbon-rich geological deposits fossil fuels.
Lately, one of humanity's very favorite pastimes is digging up
all of this old carbon in the form of coal, and oil and natural gas,
and burning it to fuel our our pretty-much-everythings.
But I'll get to that later.
Another extremely important carbon reservoir is the ocean.
Now, carbon dioxide dissolves really easily in water,
and once it's in there, a lot of it's used by phytoplankton,
tiny plant-like organisms that form the base
of the marine food chain.
They use carbon in photosynthesis and they also use it
to make calcium carbonate shells, and when these guys die,
their shells settle to the bottom of the ocean, pile up,
become compressed, and over time, make rocks like limestone.
Now, limestone obviously doesn't burn super well,
so it's not considered a fossil fuel.
But as limestone deposits are eroded by water,
the calcium carbonate is broken down to eventually form,
among other things, carbon dioxide and carbonic acid.
We make lime and cement by heating limestone,
which produces a pretty good amount of carbon dioxide.
And when we do burn fossil fuels such as coal, petroleum products,
and natural gas, it also releases carbon in the form of
carbon dioxide that's been stored for hundreds of millions of years
in the geosphere, which is just a fancy, sciency word for earth rocks.
This process is what started the atmospheric carbon dioxide
levels rising like crazy in the past couple hundred years.
And the excess of carbon dioxide in the atmosphere causes
global climate change, because CO2 in the atmosphere prevents
some of the Sun's energy from re-radiating back out into space.
So, yeah, our planet is getting warmer because we've been
burning through this massive reservoir of carbon that
we had locked underground.
This is causing all kinds of problems that we can see already,
and it's very likely going to keep causing bigger
and bigger problems with time.
And the situation could be helped a lot if we would just stop
unlocking all that carbon and spitting it into the atmosphere,
but in some respects, we don't even have control
of the situation anymore, because of ice.
Remember how I said that carbon is often trapped in ice?
Well, in places like Siberia, Northern Canada and Alaska,
cold places that also have plants, they contain huge carbon
reserves that are trapped in permafrost,
ground that's frozen year-round.
These places are basically frozen wetlands that add another
layer of dead plant matter each year.
But as permafrost melts, these dead plants decompose and
huge amounts of carbon dioxide and methane are released
into the atmosphere, creating a positive feedback loop:
our carbon-burning lifestyles unleashing this other
huge carbon reservoir, which keep the whole
greenhouse effect going, with or without us.
Just saying.
Sorry to end on such a frightening and depressing note,
but the stability of global climate...is not as stable
as we would like it to be, and the fact that we're throwing it
out of whack is one of the most important reasons
to study ecology in the first place.
This episode of crash course was written by myself,
Jesslyn Shields and Blake DePastino.
Our technical director is Nick Jenkins, who is also our editor,
and is also standing behind the camera right now.
Graphics are courtesy of Amber Bushnel and Peter Winkler
and our sound designer is Michael Aranda.
If you want to review any of what we went over in today's episode,
check out the table of contents over there,
and if you have any questions, comments,
corrections or ideas for us, we're on Facebook and Twitter
and of course, down in the comments below.
コツ:単語をクリックしてすぐ意味を調べられます!

読み込み中…

The Hydrologic and Carbon Cycles: Always Recycle! - Crash Course Ecology #8

1587 タグ追加 保存
羅紹桀 2015 年 8 月 1 日 に公開
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